810 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 7630 



within certain limits of speed, diving-plane 

 angles, fore-and-aft inclination of the vessel, and 

 excess static weight or buoyancy 



(g) Change course in a horizontal direction 

 when submerged, from the small angles involved 

 in normal steering to turns of 180 deg or more 



(h) Possess good wavegoing performance as a 

 surface vessel, including reasonable safeguards 

 for personnel who may be on deck at sea, when 

 the craft is either stopped or underway 



(i) Provide adequate freeboard in the surface 

 condition, for access hatches leading to the 

 pressure hull which may be open when underway 

 or at anchor 



(j) Rest on the bottom for appreciable periods, 

 and possibly travel along the bottom. 



II. Physical Arrangement. To serve as a back- 

 ground for a discussion of hydrodynamic design 

 problems of a submarine there must be some 

 knowledge of the principal physical features of 

 this type of vessel. It is possible but not likely 

 that a further half-century of development will 

 modify somewhat the physical arrangements of 

 the modern (1955) design. One such design, 

 relatively modern, is shown by G. de Rooij 

 ["Practical Shipbuilding," 1953, Fig. 42 on p. 29 

 and Fig. 788 m the back of the book. A brief 

 description is given in Sec. 203 on page 368]. 



The submarine which is also required to give 

 a good account of itself on the surface, called a 

 submersible in this book, possesses two rather 

 distinct hulls, taking into consideration the 

 underwater and the abovewater portions as 

 units performing distinctly different functions. 

 The always-buoyant inner or 'pressure hull is of 

 a form best adapted to resist external hydrostatic 

 pressure, with practically no regard for the ease 

 with which it could, as an independent unit, be 

 driven through or along the surface of the water. 

 The outer hull is a ship-shaped envelope built 

 around the inner or pressure hull, designed to 

 minimize resistance of the combination for surface 

 propulsion and to provide spaces between the 

 hulls for water-ballast tanks and fuel tanks. 



The portion of the outer hull lying below the 

 waterplane in surface condition, indicated in 

 diagram 2 of Fig. 37. C and in the schematic 

 section of Fig. 76.V, fulfills exactly the same 

 function for a submarine as for a surface vessel. 

 It is generally designed in the same manner, 

 with necessarily more regard for machinery 

 clearance, access between hulls, and special 



fittings. Similarly, the underwater hull design is 

 tied into the abovewater design to msure accept- 

 able wavegoing performance, the same as for the 

 surface ship. While it is perfectly possible for a 

 properly sealed submarine to plow through 

 surface waves instead of riding over them, it 

 suffers from much the same retardation as would 

 a surface vessel under similar circumstances. 



Most of the space between the inner and the 

 outer hulls is devoted to the provision of added 

 buoyancy when the vessel is on the surface. In 

 this condition the water-excluding volume of the 

 outer hull up to the surface waterline, in what is 

 known as diving trim at full buoyancy, is equal 

 to the water-excluding volume of the inner hull 

 plus all its external appendages. The result is 

 that, when the vessel is on the surface, the inner 

 hull and its appendages are raised above the level 

 of the surrounding water by a volume equal to 

 that of the main-ballast tanks, between the mner 

 and outer hulls and below the surface waterline. 

 This main-ballast-tank volume divided by the 

 inner or pressure-hull volume is thus the reserve- 

 buoyancy ratio in the surface condition. 



It is customary, on double-hulled submersibles, 

 for the main-ballast tanks to extend above the 

 surface waterline in diving trim. In fact, this is 

 usually necessary, to provide adequate initial 

 metacentric stability and a safe range of positive 

 stability. The volume of the main-ballast tanks 

 above the surface waterline adds to the reserve- 

 buoyancy volume of the pressure hull and of all 

 water-displacing appendages and objects above 

 the waterplane. The vertical hatching of Fig. 

 76.V indicates this volume in schematic fashion. 



A rather complete general discussion of these 

 features, including the matters of equilibrium 

 of static forces and of metacentric stability 

 discussed subsequently in this section, is given 

 by A. I. McKee [Bu C and R, Tech. Bull. 8-29, 

 Nov 1929; also "Development of Submarines in 

 the U.S.," SNAME, HT, 1943, pp. 344-355]. 



When the craft submerges, sea water must be 

 admitted to the main-ballast tanks to destroy 

 the buoyancy which lifted the pressure hull above 

 the water in the surface condition. For a vessel 

 having a water-excluding displacement when 

 submerged of say 3,000 tons, the weight of water 

 to be admitted to the main-ballast tanks may 

 exceed 1,000 tons. Furthermore, to permit 

 flooding with this water, the air in the main- 

 ballast tanks must be vented to the atmosphere. 



Admitting this much water and venting an 



